The present invention relates to a switching drive circuit, and more particularly, to a circuit which drives a load by switching it on and off in response to a predetermined pulse signal.
Motors and other loads can be driven by several methods, among which the two-way switching drive method is quite common. This method has the advantages of low loss and reduced power consumption, and is particularly effective for driving loads such as motors in battery-powered devices, which may be portable or can be installed on vehicles.
A configuration of a conventional two-way switching drive circuit is shown in FIG. 1A. The circuit includes a pnp transistor Q.sub.20 and an npn transistor Q.sub.21, which are connected to each other at their collectors, and a pnp transistor Q.sub.22 and an npn transistor Q.sub.23, which are also connected to each other at their collectors. A motor M is connected between the two common collector junctions.
When a negative pulse signal is applied to the base of transistor Q.sub.20 while a positive pulse signal is being applied to the base of Q.sub.23, a drive current will flow in the motor M in the direction indicated by a solid-line arrow, causing the motor to rotate in the forward direction. When, on the other hand, a negative pulse signal is applied to the base of transistor Q.sub.22 and a positive pulse signal to the base of transistor Q.sub.21, a drive current will flow in the motor M in the direction indicated by a dashed-line arrow, causing the motor to rotate in the reverse direction. As shown in FIG. 1A, one terminal of the motor M is connected to a power supply V.sub.cc via a diode D.sub.10, while the other terminal of the motor is connected to V.sub.cc via another diode D.sub.11. Both diodes serve to absorb reverse electromotive force.
In terms of an equivalent circuit, the motor M consists of two components, a resistance and an inductance. Part of the energy of the drive current flowing in the motor M is consumed in the resistance as driving energy, and the remainder is stored in the inductance. The stored energy produces a reverse electromotive force when the applied drive pulse is removed, and, after flowing through a closed loop including the diode D.sub.10 or D.sub.11, the energy will be consumed as driving energy in the resistance. Therefore, theoretically, all of the energy that is generated in the circuit will be consumed as driving energy and the circuit has a linear input vs. output relationship, as indicated by the broken line in FIG. 1B. In practice, however, the energy loss in the diode D.sub.10 or D.sub.11 distorts the ideal linear relationship, resulting in the curve shown by the dashed line in FIG. 1B. As shown in FIG. 1C, this causes the small input gain (i.e. the gain when a drive pulse of small width is applied) to be lower than the theoretical value (indicated by the dashed line).
The approximate energy loss which occurs in the two diodes due to absorption of reverse electromotive force will now be calculated. FIG. 2 is a diagram of an equivalent circuit of the switching drive circuit, and FIG. 3 shows waveforms of signals produced in response to a drive pulse (a). If it is assumed that no energy loss occurs in either of the diodes for absorption of reverse electromotive force, the discharge curve will follow the solid line (1) FIG. 3. In fact, however, the diodes cause energy losses as indicated by the solid line (2), and the discharge curve follows the dashed line (3). The energy that is actually consumed in a resistor R is represented by the hatched area W.sub.A, and the energy that is lost in the diodes due to absorption of reverse electromotive force is represented by the hatched area W.sub.B. The sum of the two areas W.sub.A and W.sub.B represents the theoretical energy consumption in resistor R (W.sub.O =W.sub.A +W.sub.B), and the efficiency .eta. is expressed by W.sub.A /W.sub.B.
Assuming a discharge current of i(t), a peak current of I.sub.L (constant) and the absence of energy loss in either of the diodes due to absorption of reverse electromotive force, the theoretical energy consumption in resistor R (W.sub.O =W.sub.A) may be calculated as follows in consideration of the relation i(t)=I.sub.L .multidot.exp(-R/L)t: ##EQU1##
Assuming that the current I is zero at time T.sub.z, that the peak current on the dashed line (3) in FIG. 5 is i.sub.p, and that the peak current caused by reverse electromotive force in the diodes is I.sub.D (a constant), W.sub.A, or the actual energy consumption in resistor R, may be calculated as follows: ##EQU2## T.sub.Z may be calculated as follows by using the relation i.sub.p =(I.sub.L +I.sub.D)(exp(-R/L)T.sub.Z)-I.sub.D. Since i.sub.p =0 at T.sub.Z, ##EQU3## Substituting Eq. (3) into Eq. (2), ##EQU4##
Further, W.sub.B, or the energy loss in the diodes due to absorption of reverse electromotive force, may be determined as follows: ##EQU5##
The efficiency, .eta., can then be determined as follows: ##EQU6##
If the number of diodes used for absorbing reverse electromotive force is n and if each diode has a forward voltage drop of V.sub.F, the following relation may be written: EQU I.sub.D =nV.sub.F /R.
Subtituting this equation and I.sub.L =(E/R)(1-exp(-R.sub.L /L)T.sub.0) into equation (6), the efficiency of a drive circuit using n diodes will be calculated as: EQU =1-(2nV.sub.F /RI.sub.L)(1+nV.sub.F /RI.sub.L .multidot.log.sub.e nV.sub.F /(RI.sub.L +nV.sub.F) (7)
If values of R=10 ohms, E=14.4 volts, L=70 H, T.sub.0 =2 sec (20 sec.times.10%), n=2, and V.sub.F =0.7 volts are substituted into equation (7), .eta. is calculated as 0.606, which means that the efficiency of the drive circuit having these characteristics is 60.6%.
If n=1 and the other conditions are the same, the efficiency is increased to 74.7%.
As will be understood from the foregoing explanation, diodes, which are essential in the switching drive circuit for absorbing reverse electromotive force, cause inevitable energy losses due to the reverse electromotive force they absorb. This energy loss is substantially constant and is small enough to be neglected if the drive pulse has a relatively great pulse width. On the other hand, if the pulse width is small, the relative proportion of the loss is increased and the efficiency of driving the load is lowered to such an extent that the desired drive energy cannot be attained.
The configuration of another prior art PWM drive circuit is shown in FIG. 4B. Two triangular wave signals, a and b, which are in phase, are produced and fed to a comparator circuit 100 as upper and lower reference inputs, with one triangular signal a being biased to a higher d.c. level than the other triangular signal b. A drive signal c is fed to the circuit 100 as the comparison input. As a result, the circuit 100 produces a pair of pulse signals d and e which have pulse widths dependent on the signal level of the drive signal and which correspond to the two directions in which the load is to be driven. The load is driven by being switched on and off in response to the pair of pulse signals d and e. The waveforms of the signals a to e are shown in FIG. 4A.
The PWM drive circuit employs a triangular generator circuit which conventionally has the configuration shown in FIG. 4C, wherein a rectangular wave signal having a predetermined period is converted to a triangular signal by an integrator 101 composed of an operational amplifier OP.sub.10, resistors R.sub.40 and R.sub.41, and a capacitor C.sub.10. It may be considered to fabricate this integrator-based triangular wave generator circuit in the form of an IC device, in which case two terminal pins P.sub.1 and P.sub.2 are necessary for making external connection to the capacitor C.sub.10. However, one of the primary requirements for a circuit configuration that is adaptive for IC device fabrication is that it have a minimum number of terminal pins.
Another difficulty with the conventional motor relates to the fact that it has a "dead zone" where it remains inactive until after the applied drive current exceeds a certain level. Due to the presence of this "dead zone", noise on the drive signal line will not cause the motor to start if the noise level is small. However, the power consumed by that current is still wasted.
When the drive circuit is operated by the power supply connected to one end thereof, the reference level of the circuit, which is set to a value determined by division of the reference supply voltage by, for example, resistors, can vary from the desired level due to factors such as variations in the circuit elements and an offset in the signal level of the drive signal. This offset voltage causes a corresponding current to flow in the motor, even if the signal level is zero, and an extra power loss occurs as in the case of noise production.
Such power loss should be eliminated since it inevitably leads to the use of a larger power unit. In addition, if the PWM drive circuit is used to drive motors in portable devices or those intended to be installed on vehicles, power consumption should be minimized since these devices use batteries as power sources. Smaller power consumption is also desirable in order to reduce the size and weight of these devices.
A circuit configuration of the power drive stage wherein the two transistors on the power supply side are configured as npn transistors is shown in FIG. 5. In this circuit, a first pair of npn transistors Q.sub.20 and Q.sub.21 are connected in series with a load, for example, motor M, between a power supply V.sub.cc and ground, and a second pair of npn transistors Q.sub.22 and Q.sub.23 are also connected in series with the motor M. The circuit further includes pnp transistors Q.sub.24 and Q.sub.25 for driving the transistors Q.sub.20 and Q.sub.22 on the power supply side. When a forward driving pulse is supplied to transistor Q.sub.24 through an inverter 20 and to transistor Q.sub.21, both transistors Q.sub.20 and Q.sub.21 are turned on to provide the motor M with a drive current flowing in the direction indicated by a solid-line arrow in FIG. 5, thereby allowing the motor M to be driven in the forward direction. On the other hand, when a reverse driving pulse is supplied to transistor Q.sub.25 through an inverter 21 and also to transistor Q.sub.23, both transistors Q.sub.22 and Q.sub.23 are turned on to provide the motor M with a drive current flowing in the direction indicated by a dashed-line arrow, thereby driving the motor M in the reverse direction. One terminal of the motor M is connected to the power supply V.sub.cc via a diode D.sub.10, while the other terminal of the motor is connected to V.sub.cc via another diode D.sub.11. Both diodes serve to absorb reverse electromotive force. In this circuit, a higher efficiency is attained with a smaller number of diodes.
Referring again to FIG. 5, transistors Q.sub.20 /Q.sub.22 on the power supply side and transistors Q.sub.21 /Q.sub.23 on the ground side are both subjected to duty-cycle control by a pulse signal. In addition, drive transistors Q.sub.24 and Q.sub.25 are provided in a stage preceding transistors Q.sub.20 and Q.sub.22 on the power supply side so to cause the transistors Q.sub.20 and Q.sub.22 to undergo a slower transition to the off state than transistors Q.sub.21 and Q.sub.23 on the ground side. If, on the other hand, in the forward driving mode, transistor Q.sub.21 were to undergo a faster transistion to the off state than transistor Q.sub.20, the energy stored in the coil of motor M would produce a reverse electromotive force which could cause current to flow through a closed loop including diode D.sub.10, thereby resulting in a consumption of driving energy in the resistance of motor M.
However, in this closed loop, the sum of the voltage drop of diode D.sub.10 and the base-emitter voltage of transistor Q.sub.20 can be regarded as V.sub.F in equation (7) above. Since this is equivalent to the presence of two diodes, a great energy loss occurs due to the reverse electromotive force, and the efficiency .eta. is only about 60.6%.